The long range objective is to define those amino acid residues of anti-nitrophenyl antibodies that specify the ligand-binding site, and those within the site that make contact with bound ligands. Definition of these residues, and their position in the linear sequence of amino acids for antibodies of diverse affinities for a particular ligand (e.g., epsilon-2,4-dinitrophenyl-L-lysine), and for antibodies of other specificities, should contribute to understanding the structural basis for antibody function and the genetic basis for the immense number of different antigens that can be bound specifically by immunoglobulins of any individual.